(1) Field of the Invention
The present invention relates to microelectronics and more particularly to multilayered thin film structures utilizing a layer of titanium nitride between layers of tantalum nitride and palladium.
(2) Description of the Prior Art
The prior art multilayered thin film structures consist of a suitable substrate (glass, ceramic, etc.) on which four layers consisting of tantalum nitride, titanium, palladium and gold are applied sequentially by deposition. The purpose of the tantalum nitride layer is to provide the resistive elements of the integrated circuit. The titanium layer is used as an adhesive layer, and the palladium layer is used as a barrier against thermal diffusion of titanium into the final gold layer which is used to provide the conductors of the integrated circuit. The desired configurations of the resistors and conductors are obtained by etching the tantalum nitride and gold layers respectively.
The etching is generally accomplished by means of chemical agents. Other methods which could be employed are ion etching or RF back sputtering etching, but these methods require techniques and equipment which are neither simple nor inexpensive and certainly not as economical as chemical etching. However, at the present state of the art no chemical etchants are known that selectively etch tantalum nitride but not titanium. As a result, if no special measures are taken, a dangerous undercutting of the titanium layer occurs in the usual photolithographic process.
In addition, the etchants for tantalum nitride etch titanium at a much higher rate then that at which they etch tantalum nitride, so that for integrated circuit geometries with very small line widths the severe undercutting of the titanium layer can cause the palladium and gold layers to peel off.
In order to avoid this peel-off, the prior art solution is to withdraw the tantalum nitride coated substrate from the vacuum station used in the deposition process; then perform the photolithography of the tantalum nitride and to reintroduce the photo engraved substrate into a second vacuum station. The deposition of titanium, palladium and gold is then performed. The structure is then withdrawn from the second vacuum station and photolithography is performed on the last three layers.
Normally this method requires two vacuum stations to be operating in parallel; one for the tantalum nitride deposition and the other for the titanium, palladium and gold deposition. This two station method is required in order to continuously perform the depositions without interruptions for the intermediate photolithography of the tantalum nitride. However, this method results in contamination of the surface of the tantalum nitride layer because of its exposure to air between the first and second vacuum cycles. This method also doubles the evacuation time. The use of tantalum as a resistive material for thin film resistors is described in Berry, Hall and Harris, Thin Film Technology, Princeton, N.J., D. Van Nostrand 1968 p. 341. The use of sequential layers of titanium, palladium and gold as a termination material for thin film resistors is also described in this reference at page 367.
Accordingly, it is an object of the present invention to provide a method of making multilayered thin film structures in a one state evacuation process. It is another object of the present invention to provide a multilayered thin film structure in which the tantalum nitride layer can be etched without undercutting the adhesive layer (titanium in prior art) between the tantalum nitride layer and the palladium layer.